Activates transcription from class II MHC promoters. Activation requires the activity of the MHC class II transactivator/CIITA. May regulate other genes in the cell. RFX binds the X1 box of MHC-II promoters (PubMed:10072068, PubMed:10725724, PubMed:9806546). May also potentiate the activation of RAF1 (By similarity). ; Isoform 2 is not involved in the positive regulation of MHC class II genes.
